Various liquid dispensers, such as positive displacement piston pumps, are used to accurately dispense viscous liquids. These pumps may be used, for example, during the manufacture of semiconductor chip packages to dispense encapsulant material around a silicone die after the silicon die has been electrically connected to a substrate, such as a PC board. For many applications, it is desirable to dispense these viscous liquids in near vacuum conditions. Vacuum encapsulation has been developed to better obtain void-free encapsulation of the silicon dies. Typically, capillary action pulls the encapsulant material beneath the die to encapsulate all of the wire bonded leads and electric contacts between the die and the substrate. Void-free encapsulation ensures that the delicate wire bonded leads are protected from moisture, chemical attack and mechanical stresses.
The pumps normally used to dispense such materials rely on atmospheric pressure to keep the liquid from dripping out of the outlet between dispensing cycles. Essentially, these pumps operate similar to syringes and dispense liquid only when a piston or plunger displaces the liquid from a supply chamber or passage through an outlet. When the piston or plunger is stopped, atmospheric pressure will keep the liquid from dripping out of the outlet. However, under near vacuum conditions these types of pumps or dispensers will drip upon shut-off and, as a result, the liquid will fall on unintended areas of the substrate or surrounding components.
During the vacuum encapsulation of semiconductor chip packages, accurate repeatability of the dispensed encapsulant material is also very important. Therefore, any valve structure used in conjunction with the dispensing pump must not have characteristics which adversely affect this accuracy and repeatability. Conventional pneumatically or electrically-actuated valves will typically have internal seals and other components that break or modify the fluid path such that highly accurate, minute amounts of liquid may be dispensed from the outlet. In these applications, it is therefore preferable that the liquid enters the valve and flows through a constant fluid path which will not change in volume, such as due to the presence of various compressible seals, even under very high pressures on the order of 500 psi to 1,000 psi and above. With specific regard to electrically-actuated valves, these valves will generate heat which is very difficult to dissipate in a near vacuum environment. Therefore, the life of any electrically-actuated valve would be significantly shortened in a near vacuum environment.
For at least the various reasons discussed above, it would be desirable to provide a highly accurate liquid dispenser which prevents dripping in near vacuum conditions but does not have other drawbacks associated with prior dispensing valves.